The present invention is directed to a method and apparatus for producing electroluminescence without a chemical reaction
It has long been known that certain metals such as aluminum (Al), magnesium (Mg), tantalum (Ta), titanium (Ti), thallium (Tl), tungsten (W), zirconium (Zr), and zinc (Zn) will emit light when anodized in certain aqueous electrolytes, such as citric acid, oxalic acid, ammonium oxalate, phosphoric acid, dilute sulfuric acid, etc. In all known instances, the significant characteristic of the electrolyte has been that it permits the formation of an oxide coating upon the metal used as the anode in the anodizing process.
The oxides of these metals are "excess" or "N-type" semiconductors. For a given alloy, the luminous intensity per watt may vary as a function of the electrolyte. For a given electrolyte, the luminous intensity per watt may vary with the impurity and its concentration in the alloy. The color of the luminescence is known to be a function of the alloy of the particular metal and independent of the electrolyte, i.e., a function of the energy given up by an excited electron as it assumes a stable state.
It is also generally known that luminescence is a property of the oxide and involves energy levels of 2.2-3.0 electron volts, as a function of the nature of the impurities in the metal. The rectifying properties of the electrolyte/semi-conductor/metal combinations result from the accumulation of space charges within the oxide and vary as a function of the type of semi-conductor and the direction of current flow. This accumulation of space charges gives rise to high electric fields within the oxides. It has been shown by Anderson (J. Appl. Physics Vol. 14, No. 601 (1943)), that the space charges within the oxide may be shifted with alternating potentials, in which event the cathodic flash augments the intensity of the anodization luminescence at certain frequencies of the alternating current.
Notwithstanding the knowledge that electroluminescence can be produced uniformly over the area of the metal during the growth of certain oxides in the presence of an aqueous solution as described above, efforts to produce an analogous electroluminescence in the absence of a chemical reaction and/or a solid state electroluminescing lamp have not succeeded. Hickmott (J. Appl. Phys., Vol. 36, No. 6, June 1965) showed electroluminescence of metal oxides in the absence of a chemical reaction, but such luminescence is non-uniform over the area of the metal oxide and exists only after breakdown of the dielectric.
The production of electroluminescence by a chemical reaction is unacceptable for applications such as lamps. In addition to the problems inherent in the handling of acids, any such reaction inevitably exhausts the reactants, may create problems in the handling of the undesired by-products of the reaction, etc.
Attempts to produce electroluminescence in the dry state are reported by Wesolowski et al., (Acta Physica Polonica, Vol. XX, No. 4 (1961)) where pure aluminum was anodized in oxalic acid by a constant current (2-5 ma/cm.sup.2) and by a constant voltage (24-240 volts d.c.), where the oxide surface thereof was coated with a transparent N-type semi-conductor (cadmium oxide CdO by cathodic reactive sputtering; and tin oxide SnO.sub.2 by chemical deposition), and where luminescence was observed upon the application of a strong electric field between the positive and negative semi-conductor electrodes. However, as cautioned therein, and as explained in Wesolowski (Acta Physica Polonica, Vol. XXIV, No. 3(9) (1963)), the electroluminescence of such metal/oxide/semi-conductor structures is dependent on the presence of liquid water (and thus a chemical reaction) as in the well known process by which aluminum is anodized in a liquid electrolyte.
It is known to produce electroluminescence in the dry state with "thick film" zinc sulfide panels. Such panels generally include a layer of copper coated phosphor grains "formed" by the passage of a d.c. current therethrough. When subjected to an electric field, certain spots on the phosphor grains luminesce. While heretofore not understood, this electroluminescence is believed to be the result of space charges at the junction of CuS (a P-type semi-conductor) on the surface of the ZnS grains (a N-Type semi-conductor).
It is also known to produce electroluminescence in the dry state with manganese doped "thin film" zinc sulfide sandwiched between dielectric layers to limit the current therethrough. Such thin film elements are disclosed, e.g., in the Takeda, et al., U.S. Pat. No. 4,394,601 dated July 19, 1983, and the Marrello, et al., U.S. Pat. No. 4,275,336 dated June 23, 1981.
Luminescence in the dry state is also disclosed in the Burmeis, Jr., et al., U.S. Pat. No. 3,406,869 dated Apr. 14, 1970 at the junction of P-type and N-type boron subphosphide B,P.
To the knowledge of applicant, electroluminescence has not previously been produced by an adherent P-type semi-conductor coating on an electrolytically grown oxide.
It is accordingly an object of the present invention to provide a novel method of electroluminescence in metal oxides by the acceleration of electrons from space charges associated with a P-N junction.
It is a further object of the present invention to provide a novel method of producing electroluminescence without any chemical reaction.
It is another object of the present invention to provide a novel method of producing electroluminescence without the decomposition of water.
It is a still a further object of the present invention to provide a novel method of producing electroluminescence in the absence of a carboxylic acid.
It is yet another object of the present invention to provide a novel solid state electroluminescing lamp.
It is still a further object of the present invention to provide a novel electroluminescing lamp using a doped metal oxide.
Yet still a further object of the present invention is to provide a novel electroluminescing lamp using an electrolytically produced metal oxide adherent to a metal substrate.
An additional object of the present invention is to provide a novel electroluminescing lamp operable at low current and power levels, with increased efficiency.
These and many other objects and advantages of the present invention will be apparent from the claims and from the following description when read in conjunction with the appended drawings.